Image intensifier devices



July 31, 1962 A. A. ROTOW 3,047,757

' IMAGE INTENSIFIER DEVICES Filed Sept. 22. 1959 x3 f f IN V EN TOR. ALEXANDER A. ROTOW AT TOE Y 3,047,7 57 IMAGE INTENSIFIER DEVICES Alexander A. Rotow, Lancaster, Pa, assignor to Radio Corporation of America, a corporation of Deiaware Filed Sept. 22, 1959, Ser. No. 841,508

3 Claims. (Cl. 313-65) This invention relates to image intensifier devices. 'In particular, this invention relates to image intensifier devices of the pickup tube type or image tube type.

Briefly, an image intensifier device is a device wherein when a light image is directed into the device, photoelectrons from a first photocathode are directed onto a phosphor screen to produce light. The light from the phosphor screen is directed onto a second photocathode which produces photoelectrons. The number of photoelectrons from the second photocathode is larger than the number of photoelectrons produced by the first photocathode so that an image is intensified.

In image intensifier devices, the photoelectrons from the first photocathode must be focused onto the phosphor screen in order to preserve picture definition. Also, the photoelectrons are accelerated to a high velocity in order to produce sufficient light upon striking the phosphor screen. One method of producing this high velocity, focused electron trajectory, is either by electrostatic focusing or by means of a combination of magnetic focusing and electric field acceleration. Electrostatic focusing often demands a very large tube and such a large tube often has bad corner resolution. In electromagnetic focusing, due to the high electron velocity, the spacing between the photocathode and phosphor screen must be large, or the magnetic field' must be strong, in order to bring the fast moving electrons into focus, or to a nodal point at the phosphor screen. When using a focusing magnetic field of desirably lower intensity, with desirably small potentials applied thereto, the requirements of focusing and acceleration are such that the length of the tube becomes longer than desirable, and may be impractical. To increase the magnetic field sufiiciently so that a desirably small or more practical tube length may be used, requires a costly, and excessively large focusing It is therefore an object of this invention to provide a new and improved image intensifying device.

It is another object in this invention to provide a novel image intensifier device characterized by its relatively short length and its use of a magnetic coil of smaller size than for prior art tubes of like length.

These and other objects are accomplished in accordance with the invention by providing an image intensifying device wherein the photoelectrons from a first photocathode pass through a low gradient electric field, in which the magnetic field is very effective and the greater part of a complete focus loop occurs; then'these photoelectrons pass through a higher gradient electric field to attain sufficient energy to produce light upon striking the phosphor screen and during which the remainder of the complete focus loop is accomplished.

The invention will be more clearly understood by reference to the following specification when read in conjunction with the accompanying single sheet of drawings, wherein the single FIGURE is a partial longitudinal sectional view of an image intensifier image orthicon in accordance with this invention.

Referring now to the single figure, the image intensifier imageor-thicon tube embodying the invention comprises an evacuated envelope 12. One end of the envelope is provided with a plurality of lead-in pins which extend through the envelope 12 and support an electron gun (not shown). The electron gun is of any wellknown type and further description thereof is not deemed 3,647,757 Patented July 31, 1962 ice necessary. The electron gun is for the purpose of producing an electron beam 20 which is accelerated by ac celerating electrodes toward a target electrode 22 that is arranged within the near end of an enlarged end portion 14 of the envelope 12. The target electrode 22 is an insulator, e.g. glass, and is supported transversely to the electron beam 2% so that the target electrode may be scanned on the surface of one side by the electron beam 2%. Mounted immediately adjacent to the target electrode 22 is a beam decelerating electrode 24, in the form of a mesh screen, which, during operation, brings the velocity of the electron beam 2% to substantially zero in front of the target surface.

On the closed, far end of the enlarged end portion 14 cf the envelope 12 there is provided a first photocathode 26 which produces photoelectrons that are directed toward a phosphor screen 28. The photocathode may be a material such as cesium-antimony while the phosphor may be a material such as zinc-cadmium sulphide. The phosphor screen 28 may be aluminized as is well-known, to prevent light from the phosphor screen 28 from feeding back onto the first photocathode 26. The phosphor screen 28 is supported on a surface of a thin transparent membrane 30, made e.g. of glass. On the opposite surface of the transparent membrane 30 there is provided a second photocathode 32, which is sensitive to limit of the Wavelength produced by the phosphor screen 28. The cesium-antimony is sensitive to the blue portion of the spectrum which is the color of the light produced by the zinc-cadmium-sulphide phosphor. Electrons from the second photocathode 32 are directed onto the unscanned side of the target 22.

. During operation of the tube 10, the electron beam 20 lands on and scans a surface of the target 22 and drives the scanned surface of the target to cathode potential. Once the scanned surface is at cathode potential, the balance of the electron beam is reflected back as a return beam 31 toward the electron gun where the beam is collected and multiplied in known manner.

When an image is to be televised, the light from the image is focused onto the photocathode 26 which produces a photo-electric image corresponding to light from the scene. The photoelectron image is focused onto the phosphor screen 2%, which produces a light image corresponding to the image from the scene. The light from the phosphor screen 28 passes through the thin membrane 30 and strikes the second photocathode 32 and thus produces a second photoelectron image that corresponds to the light from the original scene. This second photoelectron image is an intensified image in that the number of electrons from the second photocathode 32 is substantially greater than the number of electrons from the first photocathode 26.

The photoelectrons from the second photocathode 32 are accelerated and focused onto the unscanned side of the target 22 which produces, by secondary emission, a charge pattern on the scanned side of the target 22. This charge pattern corresponds to the light from the original scene being reproduced. When the electron beam now scans the target 22, the beam is reflected, as a return electron beam 31 from the areas of the target which are not charged by the photocathode 32, since these areas are at cathode potential. The electron beam 20 lands on the areas of the target 22 that have been charged by the photocathode until the charge pattern is neutralized by the electron beam 20. Once the charge pattern is neutralized, the balance of the beam is reflected back toward the electron gun. Thus, the return beam 31 is modulated in proportion to the amount of charge on the target and 7 is multiplied and [amplified to become the television output signal.

The magnetic fields which are required for the above described operation are produced by a deflection yoke 34, focus coil 36 and an alignment coil 38.

In order to operate as described above, the photoelectrons from the first photocathode 26 must be focused onto the phosphor screen 28, in order to retain picture definition. In order to provide sharp focus, these photoelectrons should complete one loop of focus around the magnetic lines of force. Also, the photoelectrons from the first photocathode 26 reach a sufficiently high velocity to penetrate the aluminum coating, if one is used, and to then bombard the phosphor screen 28 with sufiicient energy to produce light.

As is well-known, high velocity electrons are substantially unaffected by a magnetic field, due to the speed at which the electrons traverse the field, as compared to low velocity electrons. Therefore, in accordance with this invention, a fine mesh screen 40 is provided between the first photocathode 26 and the phosphor 28. The fine mesh screen 40 is supported across an opening in a hollow tubular electrode 42. The hollow tubular electrode may be in the form of a wall coating or may be in the form of a ring, as shown, supported within the enlarged end 14. Between the hollow tubular electrode 42 and the photocathode 26 is a second hollow tubular electrode 44 which may also be in the form of a wall coating if desired. With appropriate potentials applied to the electrodes, the photoelectrons from the first photocathode 26 travel at a very low velocity between the first photocathode 26 and the mesh screen 40 and are greatly influenced by the magnetic field of the focusing coil 36. The slow velocity electrons are influenced sufiiciently so that the electrons nearly complete one loop of focus at the mesh screen 40. Between mesh screen 40 and the phosphor 28 is provided a high potential gradient which accelerates the electrons substantially above the minimum velocity required to produce light. While in this area, the photoelectrons are only slightly affected by the magnetic field of the focusing coil 36 but are sufliciently affected to complete the one loop of focus that was started between the photocathode 26 and the mesh screen 40, and thus focused onto the phosphor 28.

Thus, the photoelectrons from the first photocathode 26 are accelerated through two stages. The first stage is a low gradient electric field wherein the magnetic field is particularly effective, and the second stage is a high gradient electric field wherein the magnetic field is relatively ineffective, but wherein the electrons attain sufiicient energy to penetrate the aluminum coating and produce light.

Suitable operating potentials for the tube 10, while using a magnetic flux of sufiicient strength to provide a magnetic field of approximately 60 gausses on the axis of the tube, are as follows: Cathode of the gun, at ground; beam decelerating electrode 24, plus 2 volts; second photocathode 32, minus 300 volts; mesh screen 40 and tubular electrode 42, minus 15,300 volts; tubular electrode 44 minus 15,400 volts; and photocathode 26, minus 15,600 volts.

No particular support structure is shown for the electrodes utilized in the tube since any suitable structure may be used for the purpose and many are known.

Although this invention has been described as particularly applicable to an image intensifier, image orthicon type pickup tube, it is equally applicable to an image intensifying image tube. Thus, this invention provides inexpensive means of utilizing a relatively short envelope in connection with relatively inexpensive, moderate size focusing coils to provide appropriate focus of electrons in an image intensifying type tube.

What is claimed is:

1. An image intensifier comprising an evacuated envelope, said envelope including a first photocathode for producing an electron image, a phosphor screen within said envelope and in the path of said electron image, a second photocathode within said envelope and positioned in the path of light from said phosphor screen, an apertured electrode within said envelope and positioned between said first photocathode and said phosphor screen, and magnetic focusing means around said envelope and extending at least around the region between said first photocathode and said apertured electrode.

2. An image intensifier image orthicon pickup tube comprising an elongated evacuated envelope having two ends, a first photocathode in one end of said envelope for producing an electron image, a phosphor screen supported within said envelope and in the path of said electron image, said phosphor screen being supported on one side of a transparent support, and a second photocathode on the other side of said transparent support whereby light from said phosphor screen lands on said second photocathode, target electrode means supported within said envelope and having one side thereof in the path of the electron image from said second photocathode, an electron gun in the other end of said envelope for producing an electron beam, means for scanning said electron beam over the opposite side of said target, an apertured mesh screen between said first photocathode and said phosphor screen, magnetic focusing means around said envelope at least in the region between said first photocathode and said mesh screen for focusing electrons from said first photocathode, and means for applying potentials to said tube, whereby electrons are slightly accelerated between said first photocathode and said mesh screen and are greatly accelerated between said mesh screen and said phosphor.

3. In an image intensifying device a first photocathode, a phosphor screen exposed to electrons from said first photocathode, a second photocathode exposed to light from said phosphor screen, means adjacent to said first photocathode and between said first photocathode and said phosphor screen for accelerating said electrons to a first velocity, means adjacent to said phosphor screen and between said first photocathode and said phosphor screen for accelerating said electrons to a second velocity, said second velocity being substantially greater than said first velocity, and means surrounding the region between said first photocathode and said means adjacent to said first photocathode for focusing said electrons, whereby desired focusing of said electrons during transit thereof through a relatively short distance is accomplished while said electrons have a desired velocity at said phosphor screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,258,294 Lubszynski Oct. 7, 1941 2,572,494 Krieger et a1. Oct. 23, 1951 2,777,970 Weimer Ian. 15, 1957 

